So... Assuming Hitler was an Atheist* - does this shed any light on the existence of God? Well, no.

There are a few separate points to address here, and I'll take them in turn.

If a bad person believes something, it is wrong because they are bad.

Imagine that no-one knew what water was made of, chemically. Then imagine an utter arsehole suggesting that hydrogen and oxygen played a part, and that he could prove it using reliable, empirical data.

Would the data - the real-world physical indicators of credibility - lose weight because the person presenting them was a bit of a dick? No. Those two things are as unrelated as a person's telephone number and hair colour - i.e one thing simply doesn't effect the other.

If you're unconvinced, consider the flip-side. If a really nice, charity donating, community supporting, Bible-bashing Christian attributed their honest nature to a sincere worship of what they imagine their God to be like, would it shed any light on the question of the existence of that God? Again, no - the same way that the entirely subjective cuteness of kittens doesn't either.

This is the most important bit, in my opinion. I congratulate you for being so young and already understanding this ogical fallacy when many old people still don't get it. If an evil person likes or believes something, should you ALWAYS do exactly the opposite? Just to continue with the "argumentum ad Hitlerum" (butchering Latin is fun),Hitler was a dog lover and a vegetarian. Should I stop being a dog lover, an animal lover and a vegetarian just because he was? And if some really good person believes that eating mercury is good for you, should I poison myself?

The moral goodness of a person has nothing to do with the truth of their beliefs. Also, even truly evil people do stuff which is morally right, sometimes.

Davy: Of course if Hitler's religiosity is acknowledged by believers, it is incidental to his world-view (or his take on Christianity is warped)... But when they claim he was an Atheist, his non-belief is the cause of his moral depravity.

Classic cherry-picking.

Michel: That makes sense to me. He was very much, um, resourceful - but I do think that he was likely acting, a lot of the time, in accordance with his own idea of a divine plan very real to him... He may have invaded highly religious countries, but probably either felt that they were 'doing it wrong' or any damage done was, again, in God's name (and actually good for the people he attacked).

The Catholic church at the time certainly did not think he was an atheist. They probably changed their mind when he invaded the very Catholic Poland.

Of course, those who nowadays claim atheism for Hitler are historically challenged:

Wiki sez:

Hitler saw the church as important politically, as a conservative influence on society. He felt that if the church were eliminated the faithful would turn to mysticism, which he thought would be a step backwards politically and culturally. Though he never officially left the Catholic Church, he had no real attachment to it. After leaving home he never again attended Mass or received the sacraments. He favoured aspects of Protestantism that suited his own views, and adopted some elements of the Catholic Church's hierarchical organisation, liturgy, and phraseology in his politics.

In public, Hitler often praised Christian heritage and German Christian culture, and professed a belief in an "Aryan" Jesus Christ—a Jesus who fought against the Jews. He spoke of his interpretation of Christianity as a central motivation for his antisemitism, stating, "As a Christian I have no duty to allow myself to be cheated, but I have the duty to be a fighter for truth and justice". In private, he was more critical of traditional Christianity, considering it a religion fit only for slaves; he admired the power of Rome but was hostile towards its teaching. Historian John S. Conway states that Hitler held a "fundamental antagonism" towards the Christian churches.

In political relations with the church, Hitler adopted a strategy "that suited his immediate political purposes". According to a US Office of Strategic Services report, Hitler had a general plan, even before his rise to power, to destroy the influence of Christian churches within the Reich. The report titled "The Nazi Master Plan" stated that the destruction of the church was a goal of the movement right from the start, but that it was inexpedient to express this extreme position publicly. His intention, according to Bullock, was to wait until the war was over to destroy the influence of Christianity.

Hitler admired the Muslim military tradition, but considered Arabs as "racially inferior". He believed that the Germans, in conjunction with Islam, could have conquered much of the world during the Middle Ages. Although Himmler was interested in the occult, the interpretation of runes, and tracing the prehistoric roots of the Germanic people, Hitler was more pragmatic, and his ideology centred on more practical concerns.